White House sources partly confirmed an alarming report that US government computers — reportedly including systems used by the military for nuclear commands — were breached by Chinese hackers.

“This was a spear phishing attack against an unclassified network,” a White House official told FoxNews.com. “These types of attacks are not infrequent and we have mitigation measures in place.”

A law enforcement official who works with members of the White House Military Office confirmed the Chinese attack to FoxNews.com on Monday, but it remains unclear what information, if any, was taken or left behind.

“This [White House Communications Agency] guy opened an email he wasn’t supposed to open,” the source said.

That email contained a spear phishing attack from a computer server in China, the law enforcement source told FoxNews.com. The attack was first reported by the conservative blog Free Beacon. Spear phishing involves the use of messages disguised to appear as valid; in fact, they contain targeted, malicious attempts to access sensitive or confidential information.